1. Field of the Invention
The present invention relates generally to devices and methods for forming fixed calcium phosphate films on isolating substrates, more specifically, upon semiconducting substrates. In a preferred application, the calcium phosphate film thus formed is used as a drug delivery system which effects the controlled release of a medicinally active ingredient to a targeted site over a predetermined time interval.
2. Description of the Prior Art
Techniques are known in the prior art for applying inorganic films to isolating substrates, such as silicon substrates. The previous methods have generally either involved high energy plasma deposition techniques which were complicated and expensive to implement or involved wet chemical etch type techniques. The wet chemical etch techniques in some cases resulted in contaminating the substrates to be coated, thereby impairing the function of the components to be manufactured from the substrate. See, for example, xe2x80x9cBioreactive Silicon Structure Fabrication Through Nanoetching Techniquesxe2x80x9d, Advanced Materials, 1995,7, No. 12. Dry coating methods are also known in various arts, including cathode sputtering and vapor deposition. These techniques are primarily used to deposit metal layers over the entire surface of a target substrate, sometimes with the aid of a mask whose openings correspond to the pattern to be deposited. These masks are relatively complicated to manufacture and their alignment relative to the substrate must be extremely accurate.
A need exists for a simple technique for permanently or semi-permanently afixing an inorganic film, such as a calcium phosphate film, to an isolating substrate. As will be explained, such films are biocomapatible and can be used to dispense medicinally active agents.
The controlled release of a medicinally active ingredient at a specific location or in reaction to a specific environmental condition has long been desired in the medical community. One promising method is to modify a silicon wafer base to carry or store medicine. Previous techniques for modifying the silicon wafer base to carry or store medicine have generally involved an etching process to create pores in the surface.
U.S. Pat. No. 6,033,582, issued Mar. 7, 2000, to Lee et al., shows one method for modifying a wafer base in which non-uniform relief patterns are created on the silicon by exposing the surface to a reactive plasma. The reactive plasma produces a reaction product to etch the surface of the silicon thus creating reservoirs to store the material. However, it is expensive and difficult to generate the plasma. The nature of this process may handicap the commercial viability of the technique. Another known method used generally for applying films to silicon substrates involves the two step wet chemical etch discussed above to form a film on the silicon surface. Again, this method is expensive as well as time consuming.
What is needed is a method for fabricating a thin porous layer on a silicon substrate and a method and apparatus for the timed or environmentally conditioned release of an active agent from the porous layer.
One object of the present invention is to provide a simple method and apparatus for applying an inorganic film to an isolating substrate.
Another object of the present invention is to provide a method for fabricating a thin porous film on a silicon substrate. In the preferred embodiment of the invention, a calcium phosphate film is fixed in permanent or semi-permanent fashion to a substrate.
It is another object of the present invention to provide a method and apparatus for the timed or environmentally conditioned release of an active agent from a calcium phosphate film which has been afixed upon a silicon substrate.
Another object of the invention is to provide such a method which can be implemented as a dry method, if desired. The dry method does not contaminate the substrate to be coated with wet chemicals and since no baths are utilized, there are no adverse environmental disposal problems created.
The foregoing objects are achieved by the device and method of the invention. The method of the invention provides for the selective, self-aligned deposition of a calcium phosphate film to a selected region of an isolating substrate. The isolating substrate is first exposed with calcium phosphate to form a coating upon the selected region. The coated region of the substrate is then subjected to a high voltage spark, whereby a fixed calcium phosphate film having a desired morphology and thickness is formed within the selected region of the substrate.
The outer layer of calcium phosphate is the inorganic component of bone and is anchored to the underlying substrate of silicon which is compatible with existing integrated circuit processing methods. The morphology and thickness of the calcium phosphate film can be controlled by such factors as the duration of the spark and the distance between the coated silicon substrate and an electrode or counter-electrode. The resultant layer of calcium phosphate is porous in nature and can be impregnated with medicinally useful substances which then can be subsequently released to the surroundings through electrical actuating means. The electrical actuating means can be formed integrally with the substrate by known integrated circuit manufacturing techniques.
In a preferred method of the invention, there is provided a method for the selective, self-aligned deposition of a calcium phosphate film to a selected region of a silicon substrate. The silicon substrate is first exposed with calcium phosphate to form a coating. An electrode is then positioned a predetermined distance from the coating on the substrate. A current is then generated between the electrode and the substrate by capacitively coupling an RF power source to a selected one of the electrode and coated substrate, the resulting current creating a spark which forms a fixed calcium phosphate film having a desired morphology and thickness within the selected region of the substrate.
If desired, the porous calcium phosphate film which is formed can then be impregnated with an active agent to be released over a predetermined time interval. In order to accomplish the timed release of the active agent, an electrical actuating means is provided, including as preferred components thereof a power supply, a CPU and electrical connection to the calcium phosphate film all of which components are mounted or formed upon the semiconducting substrate using known integrated circuit manufacturing techniques. The preferred electrical actuating means further includes a sensor for detecting selected environmental conditions, the sensor being electrically connected to the CPU, whereby the sensor causes the CPU to trigger a release of the active agent upon detection of a preselected environmental condition. In the preferred embodiment of the invention, the active agent is a medicament with the device containing the medicament being implanted or otherwise introduced within the body of a human being or other living creature. The electrical actuating means can be signaled to release the medicament over a predetermined time interval.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.